Telefonica currently holds around 85 percent of Spain's fibre optic network, which it is using to offer bundled packages of super high-speed internet, on-demand television and telephone.

"The CNMC's proposal to open up fibre optic across most of the country, obliges a future review of the investments and where they will be," according to a Telefonica internal report seen by Reuters on Monday.

In December, the competition watchdog CNMC proposed that Telefonica should open its fibre optic cable network across Spain except for nine major cities to ensure competition and promote investment in next-generation access networks.

Telefonica plans to cut cable placement by 35 percent, or to some 3.6 million homes this year down from the initial 5.5 million planned, as a result of the CNMC proposal, according to the document.

(Reporting by Robert Hetz, writing by Paul Day; Editing by Julien Toyer and Jane Merriman)